The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease are a few of the causes which can result in spinal pathologies for which permanent immobilization of multiple vertebrae may be necessary. A variety of systems have been disclosed in the art that achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies.
The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determines the appropriate surgical protocol and implantation assembly. Posterior fixation is much more commonly used in the lower back, i.e., the sacral, lumbar, and lower thoracic regions, than in the upper regions of the thoracic and the cervical spine. The use of screw and plate assemblies for stabilization and immobilization via lateral or anterior entrance in these upper regions is, however, common.
Because the cervical spine is routinely subject to mechanical loads which cycle during movement, one of the primary concerns of physicians performing cervical plate implantation surgeries, as well as of the patients in whom the implants are placed, is the risk of screw pull-out. This is of particular concern in the cervical region because of the critical vessels that abut the anterior surfaces of the cervical spine. Screw pullout occurs when the cylindrical portion of the bone that surrounds the inserted screw fails. A bone screw that is implanted perpendicular to the plate is particularly weak because the region of the bone that must fail for pullout to occur is only as large as the outer diameter of the screw threads. It has been found that for pull-out to occur for a pair of screws which are angled inward, “toe nailed”, or ones which diverge within the bone, the amount of bone which must fail increases substantially as compared to pairs of screws which are implanted in parallel along the axis that the loading force is applied.
It has, therefore, been an object of those in the art to provide a screw plate assembly that permits the screws to be entered into the vertebral body at angles other than 90 degrees. Certain screw plate assemblies that have been developed fix the angulation of the screw at an angle other than 90 degrees. One such screw plate assembly is the Orion® Anterior Cervical Plate System of Sofamor Danek USA, 1800 Pyramid Place, Memphis, Tenn. 38132. The Orion™ system teaches a plate having two pair of guide holes through which the screws are inserted to fix the plate to the vertebral body. The plate further includes external annular recessions about each of the guide holes that are radially non-symmetric in depth. More particularly, the annular recessions serve as specific angle guides for the screws so that they may be inserted non-perpendicularly with respect to the overall curvature of the plate. Thus, a given plate can accommodate only one screw-in angulation per hole, preferably in accordance with the angle of the annular recession. This is undesirable, in that physicians often must inspect the vertebral bodies during the implantation procedure before making the decision as to which screw-in angle is the ideal. By forcing the physician to chose from a limited set of angles, it is unavoidable that physicians will be forced to implant plates having screws which were positioned non-ideally. While providing a variety of plates having different angle guide holes and annular recession orientations is possible, the complexity and expense of providing a full spectrum of plates available in the operating room for the surgeon to choose from is undesirable. It is a failure of the system that one plate cannot accommodate a variety of different screw-in angles.
Accordingly, other screw plate assemblies that have been developed allow the screws to be angled at a plurality of angles (rather than a single angle) relative to the plate, whereby a single plate is compatible with a wide range of screw-in angles. One such screw plate assembly is the system described in U.S. Pat. No. 6,780,186 to Errico et al., which is fully incorporated herein by reference. Using the plate typically involves positioning the plate against the desired vertebral bodies and slidably positioning elongated coupling elements of the assembly in respective elongated through holes in the plate in order to align the entry points for the screws. Next, pre-drilled holes are formed in the vertebral bones at the desired positions and angles, into which the screws are to be inserted. With the plate in place, the screws are inserted through the coupling elements and the through holes, and into the vertebral bodies. As each screw is advanced into the bone, at the desired angle, the semi-spherical head of the screw advances into the interior volume of the coupling element. Continued independent advancement of the screw is prevented by the interference of the relative screw head size and the bottom opening of the coupling element. Continued advancement of the screw, however, causes the coupling element to advance deeper into the tapered through hole. As the tapered surface of the coupling element advances, the lateral constraining forces of the mutual tapers (of the coupling element and the through hole) causes the coupling element to contract slightly as the axial slot or slots are narrowed. This contraction causes the interior volume to crush-lock to the semi-spherical head of the screw thereby locking it at the given angulation and to the plate.
While such a screw plate assembly allows the screws to be angled arbitrarily, it does not assist the surgeon in forming the above-mentioned pre-drilled holes, at desired positions and angles, into which the screws are to be inserted. Accordingly, for use with such screw plate assemblies, and other assemblies within and without the spine industry, there is a need for a drill guide that assists the surgeon in this regard and achieves other desirable results not specifically stated herein.